1. Field of the Invention
The present invention relates to a semiconductor device constructed with compound semiconductors, and particularly to a semiconductor device pertaining to a planar avalanche photodiode having a multiplication layer.
2. Description of the Related Art
A conventional planar avalanche photodiode, as shown in, for example, I. Watanabe et al., “Planar-structure Superlattice APDs”, TECHNICAL REPORT OF IEICE., LQE97-79, pp. 69–74 (1997-10), has been of a construction in which formed on one surface of a semi-insulating InP substrate is an anti-reflection film (AR coat) 1, while stacked on the other surface thereof are a p type layer 3 made of InP or AlInAs with a high carrier density, a light absorption layer 4 made of p type InGaAs with a low carrier density, an electric field relaxation layer 8 made of p type InP or AlInAs, a multiplication layer 9 made of AlInGaAs/AlInAs superlattice or AlInAs as a single layer, an n type window layer (cap layer) 11 made of n type InAlAs with a high carrier density, an n type contact layer 12 made of n type InGaAs with a high carrier density and an n type side electrode 10.
In a conventional planar avalanche photodiode of FIG. 10, a groove for isolating an n type InAlAs window layer 11 (an n type cap layer 11) constituting a diode from a p type layer is formed around the n type cap layer 11, a p type high carrier density layer 5 is formed so as to be of a p type conductivity and have a high carrier density with Zn diffusion outside the groove and a p type side electrode 6 is formed thereon. A guard ring region 13 injected with Ti ions is formed directly below the groove, which reduces a junction leakage current to secure a stable operation of the diode. An n type contact layer 12 made of an n type InGaAs with a high concentration is formed between the n type cap layer 11 and an n type side electrode 10 to decrease ohmic contact resistance. The device with such a construction is protected by a surface protective film 14 and fixed with bumps 7.
Using a circuit in FIG. 11, there is shown an equivalent circuit of the conventional planar avalanche photodiode with the construction described above, and a diode D1 and resistance values R1 and R2 correspond respectively to the pn junction serving as a light receiving region, the resistance of the p type layer 3 with a high carrier density and the sum of the resistance of the Zn diffusion region 5 and the ohmic resistance of the p type side electrode 6.
Then, description will be given of workings of the construction. Light is transmitted through the AR coat 1 to come in, from the InP substrate 2 side (the rear surface side). Since the semi-insulating InP substrate 2 and the p type layer 3 with a high carrier density have large bandgaps, light with a wavelength (1.3 μm or 1.55 μm) employed in common optical communication transmits those without being absorbed therein. The transmitted light is absorbed in the InGaAs light absorption layer 4 small in bandgap to generate electrons and holes. In an operating state, a high reverse bias voltage of the order of 25 V is applied across the avalanche photodiode (APD) and depletion occurs in the light absorption layer 4, the electric field relaxation layer 8 and the multiplication layer 9. In the depletion layer, holes flow toward the p type layer 3 with a high carrier density, while electrons flow toward the multiplication layer 9 across which a high electric field is applied. In this situation, electrons trigger avalanche multiplication in the multiplication layer 9 across which a high electric field is applied to thereby generate many of new electrons and holes. As a result, an optical signal is taken out to the outside as a multiplied signal current. A signal current tens of times as large as that when no multiplication occurs can be taken out by the multiplication.
While an avalanche photodiode can be employed in optical communication at a higher bit rate with a wider response band, a response band of an avalanche photodiode is restricted mainly by three factors including “a travel time of an electron or a hole through a depletion layer”, “a time causing avalanche multiplication” and “a discharge/charge time on a circuit to be determined by a CR time constant.”
“The travel time of an electron or a hole through a depletion layer” and “the time causing avalanche multiplication” decrease with a smaller thickness value of the light absorption layer 4 or the multiplication layer 9.
Then, attention will be directed to “a discharge/charge time on a circuit to be determined by a CR time constant”; a response band fc (a cut-off frequency is a frequency at which a frequency response takes −3 dB) is given as fc=½(2πCR) using a CR time constant, and thereby with decrease in value C or R, fc is larger.
In the conventional avalanche photodiode shown in FIG. 10, however, a problem has arisen that a cut-off frequency fc cannot be sufficiently high.
Description will be given using an equal circuit of FIG. 11; a cut-off frequency fc is given fc=½(2πC0×(R1+R2). Herein, C0 is a capacitance of a pn junction of the diode D1, R1 is a resistance value of the p type layer 3 with a high carrier density and R2 is a resistance value obtained as the sum of the resistance value of the Zn diffusion region 5 and the ohmic resistance value of the p type side electrode 6. In a case of an avalanche phtodiode for 10 Gbps, a diameter of a pn junction is on the order of 45 μmφ and the capacitance C0 thereof is on the order of 0.25 pF. The resistance R2 is usually on the order of 5 Ω. In a case where a thickness of the p type layer 3 with a high carrier density is 2 μm, made of InP with a carrier density is 1×1018 cm−3, and a width of the Ti injected guard ring region is 20 μm, a resistivity of the p type InP layer with a carrier density of 1×1018 cm−3 is 0.08 Ω cm; therefore, R1 takes 56.6 Ω. If a load impedance of the avalanche photodiode is 50 Ω, fc=½(2×π×0.25 pF×(56.6+5+50))=5.7 GHz.
In FIG. 12, there is shown a result of calculation on a frequency response characteristic in a case where only the CR time constant is considered. In FIG. 12, a frequency response in a conventional example (R=56.6 Ω) is clearly lower than in an ideal case (R1=0) and a value in the conventional example does not meet the APD band fc (>10 GHz) necessary for receiving a digital signal of 10 Gbps. Incidentally, in order to meet a condition of an APD band fc (>10 GHz), it is necessary for R1 to be 8.7 Ω or less. In order to take 8.7 Ω or less for R1, a thickness of the p type layer 3 with a high carrier density is necessary to be 13 μm or more, whereas a reality negates crystal growth of such a thick layer if consideration is given to a manufacturing cost. Even if a carrier density of the p type layer 3 with a high carrier density is higher, an R1 value can be reduced, but with a higher carrier density, invalid light absorption is raised in the p type layer 3 to thereby lower the sensitivity of the APD. Moreover, a p type impurity introduced into the p type layer 3 at a high concentration in order to increase a carrier density is easy to be diffused out into peripheral layers to thereby diffuse into the InGaAs light absorption layer 4 and the InP substrate 2, which degrades a characteristic of the APD.
As a method to avoid the problem, if a polarity of n or p of conductivity types of all the layers is reversed (p type is changed to n type and vice versa), the p type layer 3 is changed to the n type layer, to thereby enable a resistance corresponding to R1 to be reduced. This method, however, cannot be applied to an avalanche photodiode with a an AlInAs layer or an AlGaAs layer as a multiplication layer. The reason therefor is because a necessity arises for electrons to be injected into the multiplication layer constituted by an AlInAs layer or an AlGaInAs layer to trigger multiplication, which necessitates the presence of a multiplication layer between an absorption layer and an n type layer, and if a polarity of p or n is reversed, a multiplication layer is, as a result, located between a p type layer and an absorption layer.